The present invention relates to safety systems and more particularly to capacitive occupancy detection devices. Occupant detection devices can be used to enable or disable a safety restraint device, such as an airbag, or to determine how many occupants are present in a vehicle or a room. These devices can be used to detect the absence of an occupant in the passenger seat of a vehicle, and thereby disable the deployment of the passenger's airbag. The number of occupants in a vehicle may also be monitored prior to an accident in order to provide a telematic unit, such as Onstar®, with an occupant count in order to dispatch an adequate amount of emergency response. Having an accurate occupant count prior to an accident can also help emergency response personnel to determine if one or several of the occupants may have been ejected from the vehicle during the collision. In the transportation industry, occupant presence detection devices can provide a quickly count of the number of passengers in a plane, train or bus. They can also show which seats are occupied and which are not. This can also apply to theaters or halls where it is desirable to know how many occupants are present and where newly arriving customers can find empty seats.
Various technologies have been proposed to sense the presence of an occupant in a vehicle. Early detection apparatus utilized one or more mechanical switches, which are actuated by the weight of the body upon the seat. Some systems use infrared or ultrasonic transmitters and receivers, which generate signals that are reflected off of the occupant and then received and processed. Capacitive sensors have also been used as a means of detecting the presence of an occupant.
Other capacitance-based systems exist that consist of only one electrode mounted between the seat foam and the seat coverings. These systems also rely on the occupant adding capacitance to the system, and thus causing a change in the voltage, current, or phase of the oscillator signal, which can be detected. However, many of these devices, which claim to be inexpensive, use circuitry that is far more complex than the circuitry of the device described herein. Some or these devices, such as the device described in U.S. Pat. No. 6,161,070, require precision power supplies and amplitude control of the waveform generated by their oscillators. They may require precision components and may only function over a small range of supply voltages. In addition, in order to provide better noise rejection, these devices must have additional circuitry to filter out noise. This adds a great deal of cost and complexity to these devices in comparison with this invention.
Furthermore, some devices, such as the device described in U.S. Pat. No. 4,796,013, cannot accurately detect whether the electrode is disconnected or damaged and will determine this situation to be an empty seat regardless of whether an occupant is present or not. This is because a disconnected electrode reduces the capacitance of the system and a capacitance below a certain threshold is assumed to mean an empty seat. This could prove to be fatal when the device is being used to provide logic that enables or disable a safety restraint device, such as an airbag.